Electoral authorities in Mexico have announced they will recount votes cast at more than half of the polling stations in Sunday's presidential election after finding inconsistencies.
Andres Manuel Lopez Obrador of the leftist PRD had demanded a total recount after initial results put him in second place.
He accused leading candidate Enrique Pena Nieto of breaking electoral rules.
Mr Pena Nieto of the PRI denied allegations his party had bought votes.
Preliminary results gave Mr Pena Nieto a lead of more than six percentage points over Mr Lopez Obrador.
Executive Secretary of the Federal Electoral Institute (IFE) Edmundo Jacobo said the recount was "an exercise in openness and transparency".
He said votes cast at 78,012 of the 143,132 polling stations would be recounted.
According to Mexico's electoral law, votes should be recounted if any of the following apply:
there are inconsistencies in the final tally reports
the result shows a difference of one percentage point or less between the first- and second-placed candidate
all the votes in one ballot box are in favour of the same candidate
On Tuesday, the candidate which preliminary results put in second place, Andres Manuel Lopez Obrador, had demanded a full recount citing evidence of widespread irregularities.
"Where these things happen, there needs to be a recount of the votes. It is not asking for a favour; it is asking for the law to be fulfilled," he said.
He has said the process itself was neither fair nor clean, accusing Mr Pena Nieto's Institutional Revolutionary party (PRI) of spending more than their allotted electoral budget and condemning biased media coverage.
Mr Pena Nieto has denied allegations of vote buying.
He told the BBC's Will Grant in Mexico City that he was "completely clear that the party acted within the law".
Asked about videos showing voters claiming they received credit with a major supermarket chain in exchange for their votes, Mr Pena Nieto said they had been faked.
He said his rival had a long record of refusing to accept defeat.
After losing the 2006 presidential election by a narrow margin to Felipe Calderon, Mr Lopez Obrador launched street protests that lasted for several months.
Preliminary results put Mr Pena Nieto on 38.15% and Mr Lopez Obrador on 31.64%.